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Abstract

Vibrational excitation near threshold was measured in cis- and trans-difluoroethene with the aim of assessing the role of the dipole moment in threshold peaks. Threshold peaks were found in both the isomer with and the isomer without a permanent dipole moment, but only for IR-active vibrations. A comparison with cross sections calculated with the Born approximation revealed that a 'nontrivial' contribution to the threshold peaks, that is, one which cannot be explained by direct dipole excitation, is found only for the isomer with a permanent dipole moment (cis).